1. Field of the Invention
The present invention relates to a high gain solid-state laser apparatus capable of producing short-pulse terahertz waves with high efficiency.
2. Technical Field of the Invention
In a solid-state laser apparatus, wavelengths available for laser amplification are determined according to the fluorescence spectral intensity of laser gain media used for the laser amplification in the apparatus. In general, when laser gain media has a broadband fluorescence spectrum (e.g. about 10 to 50 nm in a case of ytterbium-doped media), the amplified laser light has a broad bandwidth and uniform spectral intensity throughout the whole bandwidth. As to a broadband amplified laser light with a bandwidth in the order of several tens nanometer (nm: 10−9m), the duration of the amplified laser pulse can be compressed up to a femtosecond (fs: 10−15 second) region in view of the relationship of Fourier conjugation between the light pulse duration Δτ thereof and the bandwidth Δν(i.e. Δτ·Δν=constant). Thus, the amplified broadband laser pulse has been applied in various fields such as non-thermal laser beam machining, femtosecond time-resolved measurement, research in physics under high intense laser field, and so on.
Recently, it has become possible to produce short-pulse terahertz (THz: 1012 hertz) light by a non-linear optical process (e.g. difference frequency mixing) using femtosecond laser. This technology has been receiving great attention in various fields such as real-time environmental measurement and hazardous material search (refer to “Future Prospect of Terahertz Technology”, Masayoshi Tonouchi, The Review of Laser Engineering, Vol. 33, No. 12, December, 2005). To generate short-pulse terahertz waves, it is necessary to mix two short-pulse laser lights that have different wavelengths in a non-linear optical crystal.
As shown in FIGS. 5A to 5C, two short-pulse laser lights having different wavelengths are created by amplifying the broadband seed laser pulse (shown in FIG. 5A) in a laser amplifier (FIG. 5B), and cutting off the central part of the broadband spectral component of the amplified femtosecond laser light so that two parts in the spectral component of the laser pulses are extracted (FIG. 5C).
For extracting the two wavelength components, there are various methods such as a method in which a transmitted spectrum of laser light is limited by providing in a pulse compressor with a spectral mask that shields the central wavelength part of the amplified spectrum of the laser pulse(refer to, for example, A. Sugita et. al, Japanese Journal of Applied Physics Vol. 46, 226 (2007)), a method in which light is amplified while the central part of the amplified light is attenuated by means of a frequency-filtering element such as an etalon inserted in an amplifier (refer to, for example, K. Yamakawa et. al, Optics Letters Vol. 28, 2402 (2003)), and so on.
Due to insertion of frequency-dependent loss (see FIG. 5B), however, all the above mentioned methods have a problem that the gain in the apparatus as a solid-state laser apparatus is low and the production of the short-pulse terahertz waves with high efficiency cannot be expected.